A map of the mid-ocean ridge system/Wikimedia Commons

First it was Russian scientists suggesting a ripple in the earth’s crust driven by massive earthquakes in the Aleutian Islands of Alaska could  trigger methane releases to accelerate global warming in the Arctic.

Now it’s U.S. scientists hypothesizing that a slowing in the movement of tectonic plates floating atop the earth’s molten core could have sparked the global cooling that brought on the Ice Ages.

Welcome to the ever more complicated world of climate change where earth and its atmosphere meet sunlight and space.

At the December meeting of the American Geophysical Union, Colleen Dalton, a geophysicist at Brown University, floated a theory she and colleagues have developed to explain the global climate switch that triggered the cooling that sent glaciers bulldozing their way as far south as the state of Kentucky in the U.S. and buried most of northern Europe beneath ice.

Over the past 20 million years, Dalton said, the geologic evidence would indicate the production of ocean crust has slowed considerably and is down approximately 30 percent from its maximum some 15 to 19 million years ago.

Sea surface temperatures

“Here, we test the hypothesis that the global cooling of the sea surface that occurred in the late Miocene has a tectonic origin,” they reported. “Almthough spreading-rate changes (mostly slowing) and length changes (mostly shortening) on eastern Pacific spreading centers dominate changes in the global sum, spreading rates along many mid-ocean ridges had decreased from the mid-Miocene to present.

“Using a simple model of the carbon cycle, we show that these ocean-crust production changes could have triggered the global cooling.”

The new theory is not the first to attribute global cooling to the planet’s geology. University of California scientists in 2019 suggested the cooling was linked to the growth of new mountains as the planet’s plates collided millions of years ago.

The collisions uplift pieces of the oceanic crust called ophiolites. Calcium and magnesium rich rocks within the ophiolites react with carbon dioxide (CO₂) and saltwater to form limestone, which stores the would be greenhouse gas as a solid.

“Earth has a long-running carbon sequestration program,” Nicholas Swanson-Hysell, an assistant professor of earth and planetary science at the University of California Berkley told the Berkley News in 2019 when the California theory was posited. “We know that these processes keep Earth’s climate in balance, but determining what causes shifts between non-glacial and glacial climates on million-year time scales is a long-standing puzzle.”

Swanson-Hysell’s idea of balance was, of course, based on geologic time which moves slowly over the course of tens of thousands of years unlike human time which is measured in decades. And the earth’s climate over the eons appears to have operated within a broad range of normal that in some ways mimics the annual variation between summers and winters at northern latitudes.

The latest theory on what flipped the climate switch stemmed from new ocean data and reanalysis of older data.

“Dalton found crust production slowed down or stayed steady at 15 of Earth’s 16 ocean ridges, and its effect on the climate may have been stark, reported Paul Voosen at Science.

“If you dramatically slow down plate tectonics in such a short time, you can put out a lot less carbon dioxide as from volcanism,” Dalton said. She and colleagues calculated that could have produced an 18 degree (10°C) drop in temperatures during the Miocene.

Up versus down

A lot has changed since then. Since near the beginning of the Industrial Revolution, global temperatures have been creeping up rather than down. The consensus of scientific opinion is that humans, whose numbers have multiplied about eightfold and who burn CO₂-producing fossil fuels in large quantities to make their lives more comfortable, are a major contributor to the increase.

CO₂ is a colorless, so-called “greenhouse gas” that lets sunight through the atmosphere to warm the earth, but slows the loss of that heat back into space.

Some climate models now predict there could be a temperature increase of up to 9.5 degrees (5.3C) by 2100 if drastic measures aren’t taken to reduce CO₂ emissions.

Some have, however, questioned the models. University of Michigan scientists who applied the models to the estimated CO₂ levels on the planet 48 million to 52 million years ago found they predicted temperatures in the tropics topping 131 degrees (55C).

Plants can’t survive in such a climate. Photosynthesis starts to falter at temperatures above 104 degrees. But fossil evidence indicates plants thrived in the tropics 50 million years ago.

Researchers concluded that some of the latest climate models may be overly sensitive to CO₂  increases and therefore project future warming that is unrealistically high, Science Daily reported in April.

Predicting the future is hard even when the number of variables is small. Nature, unfortunately, is full of variables as the ongoing pandemic should make clear to everyone on the globe.

The Christian Bible might have promised humankind it could “replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth.”

But it hasn’t quite worked out that way. There remain many earthly events beyond human control, and a lot left for humans to learn about how exactly the planet’s complex natural systems function.

It wasn’t all that long ago that the level of solar radiation striking the planet was thought to be the main climate driver.  Solar radiation remains important, but the picture has gotten a whole lot more complicated in recent years.